1. Field of the Invention
The present invention relates to improved powder metallurgy techniques which provide fully dense electrical contact members for electrical current applications.
2. Description of the Prior Art
High density electrical contacts are well known. For example, Gainer, in U.S. Pat. No. 3,960,554, teaches mixing a minor amount of copper powder with chromium powder, pressing to form a compact, and vacuum sintering to infiltrate the chromium matrix with copper. Gainer, in U.S. Pat. No. 4,190,753, teaches a similar process, utilizing cold isostatic pressing, with minor amounts of chromium in copper powder. Hoyer et al., in U.S. Pat. No. 4,137,076, teach a contact made from Ag, WC and TiC powders, where the mixture is compacted, and then sintered at 1,260.degree. C. in a reducing atmosphere to shrink the compact. This compact is then melt infiltrated with silver, applied in the form of a slug. Kim e al., in U.S. Pat. No. 4,028,061, teach mixing silver powder with cadmium oxide powder; pressure compacting the mixture; impregnating the compact with a solution of an alkali metal salt sintering aid; heating the impregnated compact to decompose the sintering aid; and then heating up to and holding at 900.degree. C. for sintering, to produce a 99.5% dense contact.
Reid et al., in U.S. Pat. No. 4,092,157, teach mixing silver powder with cadmium oxide powder; pressure compacting the mixture; pre-heating the compact up to and holding at from 750.degree. C. to 850.degree. C. for about 1 hour; and then heating up to and holding at 900.degree. C. This appears to provide compacts of about 94% of theoretical density. This controlled thermal cycle is said to provide fine cadmium oxide distribution with minimum aggregate formation. Kim et al., in U.S. Pat. No. 4,450,204, teach making two layer contacts having a silver backing and a silver-cadmium oxide body. Here, silver powder and cadmium oxide powder are mixed and placed in a die; a mixture of silver oxide, silver acetate, and silver powder is placed in the die over the previous mixture; the material is pressed at up to 3,525 kg/cm.sup.2 (50,000 psi); and then the compact is heated up to and held at 900.degree. C.
Nyce, in U.S. Pat. No. 4,591,482, teaches the steps of: mixing metal powders specific to samarium, neodymium, cobalt, nickel, titanium, aluminum, copper, vanadium, and stainless and tool steel component powders, having a particle size below 44 microns diameter; pressing at up to 8,460 kg/cm.sup.2 (120,000 psi), to 80% of theoretical density; sintering the compact, in "green" form if self-supporting or in a sealed canister if not, at from 1,100.degree. C. to 1,370.degree. C. in a vacuum furnace, to provide 93% to 95% of theoretical density; pressurization, in the sintering chamber or in a separate chamber, up to 211 kg/cm.sup.2 (3,000 psi) at a temperature just under the original sintering temperature, with optional temperature spiking to sintering temperature; and then gradual pressure release while cooling, to provide a compact of 98% to 99.5% theoretical density. Temperature spiking can be used to compensate for the cooling effect of the compact due to introduction of cool pressurizing gas or transfer of the compact to the pressure stage. This low pressure assisted sintering (PAS) process is taught as involving less expensive equipment than hot isostatic pressing (HIP), which involves pressures of from 140 kg/cm.sup.2 (2,000 psi) to 2,115 kg/cm.sup.2 (30,000 psi) and temperatures of from 900.degree. C. to 1,360.degree.C.
Sinharoy et al., in U.S. Pat. No. 4,699,763, teach silver-graphite fiber contacts also containing up to 3 weight percent of a powdered wetting agent selected from nickel, iron, cobalt, copper, and gold. The process involves mixing the components, including a lubricant, drying, screening, pressing to 1,408 kg/cm.sup.2 (20,000 psi), heating between 120.degree. C. and 230.degree. C. in air to remove lubricant, sintering between 800.degree. C. and 925.degree. C. in a reducing atmosphere, repressing at about 7,050 kg/cm (100,000 psi), repeating the sintering step, and repeating the pressing at 7,050 kg/cm.sup.2.
All of these methods have various drawbacks in terms of providing electrical contacts having the desired properties of full density, high rupture strength, enhanced metal-metal bond, and enhanced resistance to thermal stress cracking. It is an object of this invention to provide a process that results in electrical contacts having all of these properties.